Field of Disclosure
The disclosure relates generally to the field of natural gas liquefaction to form liquefied natural gas (LNG). More specifically, the disclosure relates to the production and transfer of LNG from offshore and/or remote sources of natural gas.
Description of Related Art
This section is intended to introduce various aspects of the art, which may be associated with the present disclosure. This discussion is intended to provide a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as an admission of prior art.
LNG is a rapidly growing means to supply natural gas from locations with an abundant supply of natural gas to distant locations with a strong demand for natural gas. The conventional LNG cycle includes: a) initial treatments of the natural gas resource to remove contaminants such as water, sulfur compounds and carbon dioxide; b) the separation of some heavier hydrocarbon gases, such as propane, butane, pentane, etc. by a variety of possible methods including self-refrigeration, external refrigeration, lean oil, etc.; c) refrigeration of the natural gas substantially by external refrigeration to form liquefied natural gas at or near atmospheric pressure and about −160° C.; d) transport of the LNG product in ships or tankers designed for this purpose to a market location; and e) re-pressurization and regasification of the LNG at a regasification plant to a pressurized natural gas that may distributed to natural gas consumers. Step (c) of the conventional LNG cycle usually requires the use of large refrigeration compressors often powered by large gas turbine drivers that emit substantial carbon and other emissions. Large capital investments in the billions of US dollars and extensive infrastructure are required as part of the liquefaction plant. Step (e) of the conventional LNG cycle generally includes re-pressurizing the LNG to the required pressure using cryogenic pumps and then re-gasifying the LNG to pressurized natural gas by exchanging heat through an intermediate fluid but ultimately with seawater or by combusting a portion of the natural gas to heat and vaporize the LNG. Generally, the available exergy of the cryogenic LNG is not utilized.
A relatively new technology for producing LNG is known as floating LNG (FLNG). FLNG technology involves the construction of the gas treating and liquefaction facility on a floating structure such as barge or a ship. FLNG is a technology solution for monetizing offshore stranded gas where it is not economically viable to construct a gas pipeline to shore. FLNG is also increasingly being considered for onshore and near-shore gas fields located in remote, environmentally sensitive and/or politically challenging regions. The technology has certain advantages over conventional onshore LNG in that it has a lower environmental footprint at the production site. The technology may also deliver projects faster and at a lower cost since the bulk of the LNG facility is constructed in shipyards with lower labor rates and reduced execution risk.
Although FLNG has several advantages over conventional onshore LNG, significant technical challenges remain in the application of the technology. For example, the FLNG structure must provide the same level of gas treating and liquefaction in an area that is often less than a quarter of what would be available for an onshore LNG plant. For this reason, there is a need to develop technology that reduces the footprint of the FLNG plant while maintaining the capacity of the liquefaction facility to reduce overall project cost. One promising means of reducing the footprint is to modify the liquefaction technology used in the FLNG plant. Known liquefaction technologies include a single mixed refrigerant (SMR) process, a dual mixed refrigerant (DMR) process, and expander-based (or expansion) process. The expander-based process has several advantages that make it well suited for FLNG projects. The most significant advantage is that the technology offers liquefaction without the need for external hydrocarbon refrigerants. Removing liquid hydrocarbon refrigerant inventory, such as propane storage, significantly reduces safety concerns that are particularly acute on FLNG projects. An additional advantage of the expander-based process compared to a mixed refrigerant process is that the expander-based process is less sensitive to offshore motions since the main refrigerant mostly remains in the gas phase.
Although expander-based process has its advantages, the application of this technology to an FLNG project with LNG production of greater than 2 million tons per year (MTA) has proven to be less appealing than the use of the mixed refrigerant process. The capacity of known expander-based process trains is typically less than 1.5 MTA. In contrast, a mixed refrigerant process train, such as that of the propane-precooled process or the dual mixed refrigerant process, can have a train capacity of greater than 5 MTA. The size of the expander-based process train is limited since its refrigerant mostly remains in the vapor state throughout the entire process and the refrigerant absorbs energy through its sensible heat. For these reasons, the refrigerant volumetric flow rate is large throughout the process, and the size of the heat exchangers and piping are proportionately greater than those used in a mixed refrigerant process. Furthermore, the limitations in compander horsepower size results in parallel rotating machinery as the capacity of the expander-based process train increases. The production rate of an FLNG project using an expander-based process can be made to be greater than 2 MTA if multiple expander-based trains are allowed. For example, for a 6 MTA FLNG project, six or more parallel expander-based process trains may be sufficient to achieve the required production. However, the equipment count, complexity and cost all increase with multiple expander trains. Additionally, the assumed process simplicity of the expander-based process compared to a mixed refrigerant process begins to be questioned if multiple trains are required for the expander-based process while the mixed refrigerant process can obtain the required production rate with one or two trains. For these reasons, there is a need to develop an FLNG liquefaction process with the advantages of an expander-based process while achieving a high LNG production capacity. There is a further need to develop an FLNG technology solution that is better able to handle the challenges that vessel motion has on gas processing and LNG loading and offloading.
Once LNG is produced, it must be moved to market, typically in LNG ships. For onshore LNG facilities, the transfer of LNG to ships is done in sheltered water such as in a harbor or from berths in more mild environmental conditions. Often FLNG requires LNG to be transferred in more open water. In open water, the design solutions for LNG transfer to merchant LNG ships becomes more limited and expensive. In addition, the marine operations of tankers versus the FLNG facilities can become more complicated such as open-water berthing of a tanker either in tandem or side-by-side. Design options become more limited and often more expensive as the designed-for ocean conditions become more severe. For these reasons, there is a further need to develop an FLNG technology solution that is better able to handle the transfer of LNG in more challenging ocean or metocean conditions.
U.S. Pat. No. 5,025,860 to Mandrin discloses an FLNG technology where natural gas is produced and treated using a floating production unit (FPU). The treated natural gas is compressed on the FPU to form a high pressure natural gas. The high pressure natural gas is transported to a liquefaction vessel via a high-pressure pipeline where the gas may be cooled or additionally cooled via indirect heat exchange with the sea water. The high pressure natural gas is cooled and partially condensed to LNG by expansion of the natural gas on the liquefaction vessel. The LNG is stored in tanks within the liquefaction vessel. Uncondensed natural gas is returned to the FPU via a return low pressure gas pipeline. The disclosure of Mandrin has an advantage of a minimal amount of process equipment on the liquefaction vessel since there are no gas turbines, compressors or other refrigerant systems on the liquefaction vessel. Mandrin, however, has significant disadvantages that limit its application. For example, since the liquefaction of the natural gas relies significantly on auto-refrigeration, the liquefaction process on the vessel has a poor thermodynamic efficiency when compared to known liquefaction processes that make use of one or more refrigerant streams. Additionally, the need for a return gas pipeline significantly increases the complexity of fluid transfer between the floating structures. The connection and disconnection of the two or more fluid pipelines between the FPU and the liquefaction vessel would be difficult if not impossible in open waters subject to waves and other severe metocean conditions.
United States Patent Application Publication No. 2003/0226373 to Prible, et al. discloses an FLNG technology where natural gas is produced and treated on an FPU. The treated natural gas is transported to a liquefaction vessel via a pipeline. The treated natural gas is cooled and condensed into LNG on the liquefaction vessel by indirect heat exchange with at least one gas phase refrigerant of an expander-based liquefaction process. The expanders, booster compressors and heat exchangers of the expander-based liquefaction process are mounted topside of the liquefaction vessel while the recycle compressors of the expander-based liquefaction process are mounted on the FPU. The at least one gas phase refrigerant of the expander-based process is transferred between floaters via gas pipelines. While the disclosure of Prible et al. has an advantage of using a liquefaction process that is significantly more efficient than the disclosure of Mandrin, using multiple gas pipeline connections between the floaters limits the application of this technology in challenging metocean conditions.
U.S. Pat. No. 8,646,289 to Shivers et al. discloses an FLNG technology where natural gas is produced and treated using an FPU, which is shown generally in FIG. 1 by reference number 100. The FPU 100 contains gas processing equipment to remove water, heavy hydrocarbons, and sour gases to make the produced natural gas suitable for liquefaction. The FPU also contains a carbon dioxide refrigeration unit to pre-cool the treated natural gas prior to being transported to the liquefaction vessel. The pre-cooled treated natural gas is transported to a liquefaction vessel 102 via a moored floating disconnectable turret 104 which can be connected and reconnected to the liquefaction vessel 102. The treated natural gas is liquefied onboard the liquefaction vessel 102 using a liquefaction unit 110 powered by a power plant 108, which may be a dual fuel diesel electric main power plant. The liquefaction unit 110 of the liquefaction vessel 102 contains dual nitrogen expansion process equipment to liquefy the treated and pre-cooled natural gas from the FPU 100. The dual nitrogen expansion process comprises a warm nitrogen loop and a cold nitrogen loop that are expanded to the same or near the same low pressure. The compressors of the dual nitrogen expansion process are driven by motors that are powered by the power plant 108, which may also provide the power for the propulsion of the liquefaction vessel 102. When the liquefaction vessel 102 has processed enough treated natural gas to be sufficiently loaded with LNG, the floating turret 104 is disconnected from the liquefaction vessel and the liquefaction vessel may move to a transfer terminal (not shown) located in benign metocean conditions, where the LNG is offloaded from the liquefaction vessel and loaded onto a merchant LNG ship. Alternatively, a fully loaded liquefaction vessel 102 may carry LNG directly to an import terminal (not shown) where the LNG is offloaded and regasified.
The FLNG technology solution described in U.S. Pat. No. 8,646,289 has several advantages over conventional FLNG technology where one floating structure is used for production, gas treating, liquefaction and LNG storage. The disclosed technology has the primary advantage of providing reliable operation in severe metocean conditions because transfer of LNG from the FPU to the transport vessel is not required. Furthermore, in contrast to the previously described FPU with liquefaction vessel technologies, this technology requires only one gas pipeline between the FPU and the liquefaction vessel. The technology has the additional advantage of reducing the required size of the FPU and reducing the manpower needed to be continuously present on the FPU since the bulk of the liquefaction process does not occur on its topside. The technology has the additional advantage allowing for greater production capacity of LNG even with the use of an expander-based liquefaction process since multiple liquefaction vessels may be connected to a single FPU by using multiple moored floating disconnectable turrets.
The FLNG technology solution described in U.S. Pat. No. 8,646,289 also has several challenges and limitations that may limit its application. For example, the liquefaction vessel is likely to be much more costly than a conventional LNG carrier because of the significant increase in onboard power demand and the change in the propulsion system. Each liquefaction vessel must be outfitted with a power plant sufficient to liquefy the natural gas. Approximately 80 to 100 MW of compression power is needed to liquefy 2 MTA of LNG. The technology proposes to limit the amount of installed power on the liquefaction vessel by using a dual fuel diesel electric power plant to provide propulsion power and liquefaction power. This option, however, is only expected to marginally reduce cost since electric propulsion for LNG carriers is not widely used in the industry. Furthermore, the required amount of installed power is still three to four more times greater than what would be required for propulsion of a conventional LNG carrier. It would be advantageous to have a liquefaction vessel where the required liquefaction power approximately matches or is lower than the required propulsion power. It would be much more advantageous to have a liquefaction vessel where the liquefaction process did not result in a need for a different propulsion system than what is predominantly used in conventional LNG carriers.
Another limitation of the FLNG technology solution described in U.S. Pat. No. 8,646,289 is that the dual nitrogen expansion process limits the production capacity of each liquefaction vessel to approximately 2 MTA or less. Although overall production can be increased by operating multiple liquefaction vessels 102, 102a, 102b simultaneously (FIG. 1), this option increases the number of ships and turrets needed for the operation. It would be much more preferable to outfit each liquefaction vessel with a liquefaction process capable of higher LNG production capacity while maintaining the compactness and safety benefits of the expander based process. A liquefaction vessel with an LNG storage capacity of 140,000 cubic meters (m3) can support a daily LNG stream resulting in an annual production of approximately 6 MTA at a liquefaction vessel arrival frequency of 4 days.
Still another limitation of the FLNG technology solution described in U.S. Pat. No. 8,646,289 is that the technology has the disadvantage of requiring frequent startup, shutdown and turndown of the liquefaction system of the liquefaction vessel. The dual nitrogen expansion process has better startup and shutdown characteristics than a mixed refrigerant liquefaction process. However, the required frequency of startup and shutdown is still significantly greater than previous experience with the dual nitrogen expansion technology at the production capacities of interest. Thermal cycling of process equipment as well as other issues associated with frequent startups and shutdowns are considered new and significant risks to the application of this technology. It would be advantageous to have a liquefaction process that can be easily and rapidly ramped up to full capacity. It would also be advantageous to limit thermal cycling by maintaining the cold temperatures of the liquefaction process equipment with very little power use during periods of no LNG production.
Yet another limitation of the FLNG technology solution described in U.S. Pat. No. 8,646,289 is that the required power plant and liquefaction trains for this technology are expected to significantly increase the capital and operational cost of the liquefaction vessel over the typical cost of a conventional LNG carrier. As stated above, the power plant required for liquefaction will need to be three to four times greater than what is needed for ship propulsion. The liquefaction trains on the liquefaction vessel are similar to what is on a conventional FLNG structure. For this reason, outfitting each liquefaction vessel with its own liquefaction trains represents a significant increase in capital investment of liquefaction equipment compared to conventional FLNG structures. This technology limits the impact of the high cost of the liquefaction vessel, by proposing an LNG value chain where the loaded LNG liquefaction vessel moves to an intermediate transfer terminal where it offloads the LNG on to conventional LNG carriers. This transport scheme shortens the haul distance of the liquefaction vessel and thus reduces the required number of these vessels. However, it would much more preferable to have liquefaction vessels of sufficiently low cost that it would be economical to haul the LNG to market without having to transfer its cargo to less expensive ships.